The diorite porphyry laccolithic intrusion of Mount Hillers, southern Henry Mountains, Utah, is surrounded by upturned beds of strata and interleaved diorite sills dipping 75°–85°.
Oligocene
History of geosciences
Tectonics
Colorado Plateau, United States of America
37°53’16”N, 110°41’51”W
The diorite porphyry laccolithic intrusion of Mount Hillers, southern Henry Mountains, Utah, is surrounded by upturned beds of strata and interleaved diorite sills dipping 75°–85°.
Mountain building is a seminal research topic of tectonics with great significance for society, providing dramatic landscapes with profound effects on ecosystems, climate, and human activities. The dome-like structures in Triassic to Jurassic sedimentary rocks of the Henry Mountains offer the defining locality (Gilbert, 1877) and one of the best exposures of laccoliths worldwide. Gilbert’s scientific methodology, developed while he investigated these structures, combines field observations with mechanical principles to set the example for those who followed (Pollard and Johnson, 1973; Jackson and Pollard, 1990), but it also provides an educational template for students of structural geology (Pollard and Martel, 2020).
During exploration of the Henry Mountains in 1875, G.K. Gilbert (1843-1918) postulated a process through which magma rose through earth’s lithosphere in a dike, spread laterally as a sill, and opened a thick chamber by lifting and bending the overlying strata. He gave these chambers of ‘molten rock’ the name “laccolite” (now, laccolith) and described associated dikes, uplifted sills, and domed sedimentary strata (Gilbert, 1877; Hunt, 1988). Geological maps of the southern Henry Mountains (Mount Holmes, Mount Ellsworth, and Mount Hillers) show the laccolithic intrusions are circular in plan and formed at similar depths, 3-4 km. The overlying sedimentary rock domes have similar radii, 5–7 km, but their amplitude increases from 1.2 to 1.8 to 2.5 km, respectively. Limb dips steepen from 20° to 50°–55° to 75°–85°, and thus record serial stages in the deformation of strata over the growing laccolithic intrusions (Jackson and Pollard, 1988). The Henry Mountains are the original and now classic examples of magmatic structures building mountains within a broader typology of mountain belts worldwide. Gilbert’s investigation exemplifies a scientific methodology that progresses from field observations to idealizations to physical principles to mechanical models to solutions tested using field data (Pollard and Martel, 2020).
G.K. Gilbert proposed a mechanical model for laccoliths and tested it with field data (Gilbert, 1877). Others followed with computer modeling of faulting and fracturing over laccoliths (Jackson and Pollard, 1988, 1990). Doming over laccoliths is cited as a canonical problem for bending sedimentary strata (Pollard and Martel, 2020).
Gilbert, G.K. (1877) Report on the geology of the Henry Mountains, Monograph. U.S. Government Printing Office. Available at: https://doi.org/10.3133/70039916.
Hunt, C.B. (1988) Geology of the Henry Mountains, Utah, as recorded in the notebooks of G. K. Gilbert, 1875–76. Geological Society of America. Available at: https://doi.org/10.1130/MEM167.
Jackson, M.D. and Pollard, D.D. (1988) ‘The laccolith-stock controversy: New results from the southern Henry Mountains, Utah’, GSA Bulletin, 100(1), pp. 117–139. Available at: https://doi.org/10.1130/0016-7606(1988)100<0117:TLSCNR>2.3.CO;2.
Jackson, M.D. and Pollard, D.D. (1990) ‘Flexure and faulting of sedimentary host rocks during growth of igneous domes, Henry Mountains, Utah’, Journal of Structural Geology, 12(2), pp. 185–206.
Pollard, D.D. and Johnson, A.M. (1973) ‘Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation’, Tectonophysics, 18(3), pp. 311–354. Available at: https://doi.org/10.1016/0040-1951(73)90051-6.
Pollard, D.D. and Martel, S.J. (2020) Structural Geology: A Quantitative Introduction. Cambridge University Press.
Marie D. Jackson.
Department of Geology and Geophysics, University of Utah, USA.
David D. Pollard.
Department of Earth and Planetary Sciences, Stanford University, USA.